How Mortgage Repayments Are Used For Tax Calculations

[CW1]

Have been told that before April 2017 mortgage payments for a buy to let mortgage could be used as an expense for tax purposes. Since then, the amount that can be declared as an expense has reduced 25% each year, meaning mortgage payments can no longer be claimed as an expense when a property is rented out !
Is this right ? Meaning the mortgage payments (higher than ever !) are actually taxed as income !

https://www.gov.uk/guidance/changes-to-tax-relief-for-residential-landlords-how-its-worked-out-including-case-studies

Each year, as the "Percentage of finance costs deductible from rental income" has gone down, the "Percentage of basic rate tax reduction" has gone up.
What does that mean ?

[ToraToraTora]

BTL mortgage payments are not "taxed as income" they must be paid from taxed income. They are no longer an allowable expense.

[CW1]

I know the payments aren't taxable income as such, but by not being able to deduct them from income, that amount becomes part of taxable income. Doesn't it ?
Say income was £20k pa, mortgage £5k pa (figures plucked out of thin air), it used be £15k would be taxable (forget personal allowances for now). Now it'd be £20k. Would it ?

My question is more about the "Percentage of basic rate tax reduction", don't know how - literally - it's part of the equation.

1st to admit I know virtually *nothing* about stuff like this !

[ToraToraTora]

it's your terminology that's flawed you are not paying tax on payments you are paying tax on taxable income. The mortgage payments are no longer deductible from the income you receive from the property before it is included in other income you may have, Other expenses, eg repairs etc are. The tax you pay depends on other income too and what tax brackets you span without knowing your full figures I cannot tell you.